1. Field of the Invention
The present invention relates to an organic light-emitting device and an organic light-emitting display using the same.
2. Discussion of the Related Art
An organic light-emitting diode (OLED) is an active light-emitting device that excites a phosphor and emits light by a recombination of electrons and holes. An organic light-emitting display including the organic light-emitting diode may be used in a wall mounted device or a portable device due to its fast response speed, low direct-current driving voltage, and ultra thinness, in comparison to a passive light-emitting device needing a separate light source such as a liquid crystal display.
The organic light-emitting diode produces a color using pixels where red, green, and blue sub pixels combine to a color. In a method of driving the subpixel, the organic light-emitting diode may be classified into a passive matrix organic light-emitting diode (PMOLED), and an active matrix organic light-emitting diode (AMOLED) employing a driving method using a thin film transistor (TFT).
The driving method of the active matrix organic light-emitting diode (AMOLED) may be classified into a current driving method, a voltage driving method, and a digital driving method.
FIG. 1 is an equivalent circuit diagram illustrating a conventional current driving active matrix organic light-emitting device (AMOLED), and FIG. 2 is a driving timing diagram of FIG. 1.
Referring to FIG. 1, the conventional organic light-emitting device 50 includes a first TFT (M2), a second TFT (M3), a first switch (S/W5), a second switch (S/W6), a storage capacitor (Cst), and an organic light-emitting diode (OLED).
The first and second TFTs (M2 and M3) have a mirror structure to supply a constant current to the organic light emitting diode (OLED), are connected at their sources with a source voltage (VDD), and are connected at their gates to the storage capacitor (Cst). The drain of the first TFT (M2) connects to the organic light emitting diode, and the drain of the second TFT (M3) connects between the first and second switches (S/W5 and S/W6).
Referring to FIGS. 1 and 2, the first and second switches (S/W5 and S/W6) are series connected between the gates of the first and second TFTs (M2 and M3) and a data line. The first and second switches (S/W5 and S/W6) are connected at their gates with a scan line, and switch the data signal (data n) by the scan signal (scan n) of FIG. 2 applied through the scan line.
The storage capacitor (Cst) is between the gates of the first and second TFTs (M2 and M3) and the second switch (S/W6), and stores the data voltage from the source voltage (VDD) by the data signal (data[n]) of FIG. 2.
The organic light-emitting diode (OLED) emits light by a current generated from the first TFT (M2) driven by the data voltage stored in the storage capacitor (Cst). A gray level of the organic light-emitting diode (OLED) is determined by the amount of the signal current. For a high gray level, a larger signal current is supplied to the organic light-emitting diode (OLED), and for a low gray level, a smaller signal current is supplied to the organic light-emitting diode (OLED).
However, the conventional organic light-emitting device has a drawback in that when a low gray level is displayed, in comparison a current supplied from a data driver is dozens of nA. Then the storage capacitor cannot be charged with a desired data voltage due to a data line load on the data line between the data driver and a pixel before the storage capacitor of the pixel is charged with the desired data voltage.
In other words, the conventional organic light-emitting device has a drawback in that when a low gray level is displayed, due to the data line load, the storage capacitor (Cst) cannot be sufficiently charged with a current of dozens of nA during a gate on time of several msec.
As a result, the conventional organic light-emitting display having a pixel circuit including the organic light-emitting device has a drawback in that it cannot be put to practical use due to the deterioration of the capability to display a low gray level.